Fig. 18 in Reproductive biology, embryonic development and matrotrophy in the phylactolaemate bryozoan Plumatella casmiana

Fig. 18 Ultrastructure of the anterior hemisphere of a bilayered embryo and embryo sac shortly before bud formation in Plumatella casmiana (TEM). a Overview of embryonic epithelia and embryo sac. The embryo shows high prismatic ectoderm and flattened mesodermal cells. Ectodermal cells possess filiform extensions on their lateral sides. b Detail of an embryonic ectodermal cell showing filiform processes, mitochondria, numerous free ribosomes and a multivesicular body in the apical part of the cell. c Detail of a mesodermal cell of the embryo sac. In contrast to the embryonic cells, it shows a lower density of ribosomes, but much more vacuoles and endoplasmatic reticulum. Presumed site of exocytosis towards the lumen of the embryo sac is marked by an arrow. d Detail of the mesoderm of the embryo including large vacuoles with unknown content (asterisks). Arrow shows an extension of the cell membrane towards the basal lamina. Abbreviations: bl basal lamina, eme embryonic ectoderm, emm embryonic mesoderm, er endoplasmatic reticulum, esm mesoderm of the embryo sac, mi mitochondria, mvb multivesicular body, nu nucleus, nuc nucleolus, ri ribosomes